1. Field of Invention
The invention relates to a process for forming a polyester toner, and more specifically to an aggregation and coalescence process for the preparation of polyester toner compositions. More in particular, the invention relates to a method of preparing a polyester toner composition by the emulsion aggregation technique in which the aggregating agent is added into the composition in stages instead of at a constant addition rate throughout the particle aggregation step.
2. Description of Related Art
In reprographic technologies, such as xerographic and ionographic devices, toners with volume average diameter particle sizes of from about 9 microns to about 20 microns are effectively utilized. However, in xerographic technologies, such as the high volume Xerox Corporation 5090 copier-duplicator, high resolution characteristics and low image noise are highly desired, and can be attained utilizing smaller sized toners having, for example, a volume average particle diameter of from about 2 to about 11 microns and preferably less than about 7 microns, and with a narrow geometric size distribution (GSD) of from about 1.1 to about 1.3. Additionally, in xerographic systems wherein process color is utilized, such as pictorial color applications, small particle size colored toners, preferably of from about 3 to about 9 microns, are desired to avoid, or minimize paper curling. Also, it is preferable to select small toner particle sizes, such as from about 1 to about 7 microns, and with higher colorant loading, such as from about 5 to about 12 percent by weight of toner, such that the mass of toner layers deposited onto paper is reduced to obtain the same quality of image and resulting in a thinner plastic toner layer on paper after fusing, thereby minimizing or avoiding paper curling.
Numerous processes are known for the preparation of toners, such as, for example, conventional polyester processes wherein a resin is melt kneaded or extruded with a pigment, micronized and pulverized to provide toner particles of the desired volume average particle diameter and geometric size distribution. In such processes, wherein large materials are mechanically reduced in size to achieve the desired smaller toner particles, it is usually necessary to subject the aforementioned toners to a classification procedure such that the desired size and geometric size distribution is attained. Also, in the aforementioned conventional process, low toner yields after classification may be obtained. For example, during the preparation of toners with average particle size diameters of from about 11 microns to about 15 microns, toner yields range from about 70 percent to about 85 percent after classification, and during the preparation of smaller sized toners with particle sizes of from about 7 microns to about 10 microns, lower toner yields may be obtained after classification, such as from about 50 percent to about 70 percent.
As an improvement to the foregoing mechanical reduction processes, processes are known in which the toner is achieved via aggregation as opposed to particle size reduction. For example, emulsion/aggregation/coalescing processes for the preparation of toners are illustrated in a number of Xerox patents, the disclosures of which are totally incorporated herein by reference, such as U.S. Pat. Nos. 5,290,654, 5,278,020, 5,308,734, 5,370,963, 5,344,738, 5,403,693, 5,418,108, 5,364,729, and 5,346,797. Also of interest may be U.S. Pat. Nos. 5,348,832, 5,405,728, 5,366,841, 5,496,676, 5,527,658, 5,585,215, 5,650,255, 5,650,256 and 5,501,935. In these methods, the toners are formed chemically in situ and do not require known pulverization and/or classification methods.
Illustrated in U.S. Pat. No. 5,593,807, the disclosure of which is totally incorporated herein by reference in its entirety, is a process for the preparation of toner compositions comprising, for example, (i) preparing an emulsion latex comprised of sodio sulfonated polyester resin particles of from about 5 to about 500 nanometers in size diameter by heating the resin in water at a temperature of from about 65xc2x0 C. to about 90xc2x0 C.; (ii) preparing a pigment dispersion in water by dispersing in water from about 10 to about 25 weight percent of sodio sulfonated polyester and from about 1 to about 5 weight percent of pigment; (iii) adding the pigment dispersion to the latex mixture with shearing, followed by the addition of an alkali halide in water until aggregation results as indicated, for example, by an increase in the latex viscosity of from about 2 centipoise to about 100 centipoise; (iv) heating the resulting mixture at a temperature of from about 45xc2x0 C. to about 55xc2x0 C. thereby causing further aggregation and enabling coalescence, resulting in toner particles of from about 4 to about 9 microns in volume average diameter and with a geometric distribution of less than about 1.3; and optionally (v) cooling the product mixture to about 25xc2x0 C. and followed by washing and drying. The sulfonated polyesters of this patent may be selected for the processes of the present invention.
In U.S. Pat. No. 5,290,654, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for the preparation of toners comprised of dispersing a polymer solution comprised of an organic solvent and a polyester, and homogenizing and heating the mixture to remove the solvent and thereby form toner composites. The appropriate polyesters of this patent may be selected for the processes of the present invention.
U.S. Pat. No. 5,945,245, the disclosure of which is totally incorporated herein by reference in its entirety, describes a surfactant free process for the preparation of toner comprising heating a mixture of an emulsion latex, a colorant, and an organic complexing agent. The process may comprise (i) preparing an emulsion latex comprised of sodio sulfonated polyester resin particles of from about 5 to about 300 nanometers in size diameter by heating the resin in water at a temperature of from about 65xc2x0 C. to about 90xc2x0 C.; (ii) adding with shearing to the latex a colorant dispersion containing from about 20 to about 50 percent of predispersed colorant in water and with a mean colorant size range of from about 50 to about 150 nanometers, followed by the addition of an organic complexing agent; (iii) heating the resulting mixture at a temperature of from about 45xc2x0 C. to about 65xc2x0 C. thereby causing aggregation and enabling coalescence, resulting in toner particles of from about 2 to about 20 microns in volume average diameter; and (iv) cooling the toner product mixture followed by isolation, and drying. The organic complexing agent may be 1,4-diaminobutane, 1,4-diaminocyclohexane, 1,7-diaminoheptane, 1,6-diaminohexane, 1,2-diamino-2-methylpropane, 1,9-diaminononane, 1,8-diaminooctane, 1,5-diaminopentane, 1,2-diaminopropane, 1,3-diaminopropane, 1,3-diamino-2-hydroxypropane, ethanolamine, triethylamine, or tripropylamine.
Methods for forming toners by the aggregation process have included the method wherein during the aggregation step, the aggregating agent is added via a slow, steady continuous addition. Such process achieves toner particles of the desired size and geometric size distribution, but takes a substantial amount of time to complete. For example, particle growth times of from about 11 to about 15 hours are needed.
What is still needed is an improved process for forming toner particles in a much shorter amount of time, thereby improving the economic feasibility of the process, and with less total coarse particles and fouling.
It is thus an object of the present invention to develop an improved method for making toner particles in which the time for obtaining the particle growth is significantly reduced over similar known aggregation processes.
It is a still further object of the present invention to develop a method of forming toner particles in which a narrow geometric size distribution is achieved with less total amount of coarse particles.
It is a still further object of the present invention to develop a method of forming toner particles that results in little or no fouling.
These and other objects of the present invention are achieved by the present invention, wherein a two stage addition of an aggregating agent is effected during the aggregation of the toner particles. In particular, an amount of the aggregating agent is first rapidly introduced into a resin/colorant mixture, and then a remaining portion of the aggregating agent is added at a slower continuous addition rate.
In a first embodiment, the invention relates to a process for forming toner particles, comprising aggregating the toner particles by adding an aggregating agent to a latex emulsion of resin having a colorant mixed therein, the adding being done under adding conditions of agitation and a temperature above a glass transition temperature of the resin, wherein the adding of the aggregating agent comprises first introducing at least about 40% by weight of a total amount of aggregating agent to be added at a rapid introduction rate and subsequently adding a remaining portion of the aggregating agent at a continuous addition rate slower than the rapid introduction rate, and subsequently cooling a product mixture obtained to a temperature below the glass transition temperature of the resin.
In another embodiment, the invention relates to a process for forming toner particles, comprising
(i) preparing a latex emulsion of resin particles by heating the resin in water at a temperature above a glass transition temperature of the resin,
(ii) preparing a colorant dispersion by dispersing a colorant in water,
(iii) adding the colorant dispersion to the latex emulsion with agitation,
(iv) subsequently adding an aggregating agent to the latex emulsion of resin having the pigment dispersion mixed therein, the adding being done under adding conditions of agitation and a temperature above a glass transition temperature of the resin, wherein the adding of the aggregating agent comprises first introducing at least about 40% by weight of a total amount of aggregating agent to be added at a rapid introduction rate and subsequently adding a remaining portion of the aggregating agent at a continuous addition rate slower than the rapid introduction rate, and
(v) subsequently cooling a product mixture obtained to a temperature below the glass transition temperature of the resin.